1. Field of the Invention
The present invention relates to an improved mass flow controller comprising a mass flow meter portion for measuring a flow rate of the fluid and a fluid-controlling portion for controlling the flow rate of fluid provided between a fluid inlet and a fluid outlet formed in a base member.
2. Description of Related Art
In a fluid-controlling portion (control valve) of the conventional mass flow controller, as shown in, for example, FIG. 5, a valve seat 54 is provided between a fluid inlet 52 and a fluid outlet 53 which are formed in a base member 51. A valve body 55 having a pointed end portion 55A shaped like a frustum of cone is driven by means of, for example, a thermal type valve seat-driving portion 56 to suitably regulate the openness of a valve mouth 57 formed in said valve seat 54.
However, in the mass flow controller having the above-described construction, since the pointed end portion 55A of the valve body 55 has a frustum of conical shape, the pointed end portion 55A cannot be brought into close contact with a wall surface of the valve mouth 57 so that a fluid may not be leaked from the valve mouth 57. Accordingly, the control of remarkably small flow rates is limited, that is, the lower limit is at most about several percent of the largest controlled flow rate. For example, a minute flow rate, such as 0.1% of the largest controlled flow rate, could not be controlled.
Even if the mass flow controller can control a minute flow rate, if the noise level in the mass flow meter portion for measuring the flow rate of fluid is high, the minute flow rate cannot be controlled with high accuracy.
The mass flow meter portion of the conventional mass flow controller has been adapted to detect a mass flow rate of a fluid flowing through the conduit on the basis of a difference between electrical power required for holding two resistors wound around the conduit, through which the fluid is flown, independent of each other at appointed temperatures, respectively. With the conventional mass flow controller, the direction of the electrical current flowing through said resistors have not been clearly defined, so that an output signal having a high noise level, i.e., an output signal having a bad signal-to-noise ratio, could be obtained due to the mutual electromagnetic induction between the resistors. In order to reduce the noise until the noise level does not come into practical question as the mass flow controller, it is sufficient to provide a noise filter in a signal-treating portion, but this spoils the control response.